One of the beauties of showing ducks is that they're so easy to prepare for a show. Ducks, housed properly, keep themselves clean so there's no need to bath them.
The only thing I ever did was polish their bills, legs & feet with oil. I used baby oil but have seen VetRx & Bag Balm used as well. After polishing the bills, legs & feet I would rub the feathers with a piece of silk cloth to bring out the sheen.